Inside a Triplett VTVM

[John]

I bought this for the leads, figuring if the meter read voltages I'd be ahead (and I love these old meters). Anyway, when I opened it up found a pair of batteries I've never seen before, not even in pictures! Figured PRR would have a bit of history on these old things.

[PRR]

Not a VTVM.

[John]

It doesn't plug in, I know. It does have a tube in it, but I couldn't imagine those batteries running a tube for any length of time.

[sluckey]

Is that a Triplett 631? I haven't seen one since the early '70s. Those Burgess XX15 batteries are rated for 22.5v and were connected in series to power the 1R5 tube in VTVM mode and the high ohms scales in VOM mode. Battery life was surprisingly long. You're unlikely to find fresh replacements. There is a 22.5 volt battery #413 that can likely be adapted but they're very pricey. I need one for my Triplett 630 VOM. There is some info floating around the net about substituting modern batteries in the 631.

Here's a manual for the 631 in case you want to try to make it work. Some of the VOM modes (voltage, current) should work without any batteries. And the X1, X10, X1K ohm ranges are powered with a D cell, so they will function without the XX15 batteries too. Good find and good luck.

http://bama.edebris.com/download/triplett/631/Triplett%20631%20VTVM_VOM%20Manual.pdf

[John]

Sluckey, thanks for the manual! yes, it is a 631. I'm not sure how accurate it might be anymore with those old resistors, but  might just give a try restoring it. I did some googling, and one fellow had installed 9V batteries to make up the 45V total and said it worked fine. If I did anything long term I'd probably try building a 45VDC power supply.

The batteries are not available in any form. Apparently the factory that made them in the 70s was closed down for toxic waste. I'm surprised that cosmetically they are in such good shape, no leaking all over or anything like that. The Dcell holder seems pretty clean, so I think I'll play with it some this afternoon if I get time. Now I'm anxious to see how it measures the lower ohms and stuff.

[John]

Well, I'm not messing with anything inside. It measures ohms right on the money, same for DC volts, read right where they should. Next time I have an amp on the bench I'll test some cathode voltage readings with it against my DMM. In the process of gluing the pieces of wood back inside the leather case where they belong, and will probably replace the wires on the test leads due to crackly insulation.

[PRR]

> how accurate it might be anymore with those old resistors

The resistors are either burned-up from heavy abuse, or they are spot-on.

> couldn't imagine those batteries running a tube for any length of time.

Is (I was wrong) a beach-radio tube. Six of those in a radio did not play all day, but with just the one you get runtime near 20 hours for the heater battery, probably longer on the B battery if you could get those things today. Yes, a stack of 9V, or the 12V dog-collar batteries, will give working life so long it would be silly (and perhaps dangerous) to fool with wall-power.

It is a "VTVM" only for DC volts and maybe the highest Ohms range. That's useful. Given a choice, the "real VTVMs" are handy for high-impedance AC readings.

In passive mode, it is basically a 50uA (0.050mA) meter. If you use it on a tube circuit flowing 5mA, loading is negligible. At 1mA (typical in gitar amps) the loaded voltage will be down enough to notice. However many older repair schematics show voltages as-measured with a 20K/V (50uA) voltmeter.

[John]

will give working life so long it would be silly (and perhaps dangerous) to fool with wall-power.

In passive mode, it is basically a 50uA (0.050mA) meter. If you use it on a tube circuit flowing 5mA, loading is negligible. At 1mA (typical in gitar amps) the loaded voltage will be down enough to notice. However many older repair schematics show voltages as-measured with a 20K/V (50uA) voltmeter.

More good info, thanks.

[sluckey]

Here's a reasonably priced 22.5v battery. You'll have to solder it/them in. Then you can use the high ohm function. May be even enough juice to operate the VTVM mode. I ordered one for my 630.

http://www.amazon.com/gp/product/B008FPCWTO/ref=ox_sc_act_title_3?ie=UTF8&smid=AESL5KIUO5ZON

[John]

Hey, thanks for the link. I just ordered a 22.5
http://www.ebay.com/itm/180956618265?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649#ht_500wt_1361

for a Simpson 262 I picked up that also works great. Nice to have a cheap alternative for the Triplett!

Now I just have to find what is keeping a Simpson 260-6p from reading anything at all. I'm afeared (after some searching) that the circuit board might be the culprit (due to corrosion from the battery that was in it when I got it). But will check all the resistors first, of course. And there are a couple diodes in there too.  Shame, because everything else is in really good shape.

I'm not buying any more meters, no matter how cheap they are!!